Backer listed recommended contact times for a halogen based on temperature and halogen concentration. The author is from the California Department of Health Services.
Halogen 2 ppm
Halogen 4 ppm
Halogen 8 ppm
These times are higher than usual recommendations to take into account any uncertainty about residual halogen and the time to kill Giardia cysts in very cold water.
If the data at 5°C and 25°C is analyzed, it can be seen that the contact time at 4 ppm is 3 times the time at 8 ppm, and the contact time at 2 ppm is 4 times the concentration at 8 ppm. This relationship is not seen with the 15°C data.
If the data at 8 ppm is analyzed then time at 15°C is twice as much and at 5° is four times as much.
All of this suggests 2 alternatives to the 15°C contact times to maintain these relationships:
If option 1 is combined with the data at 5°C and 25°, then
contact time at 5°C =
= 300 - 30 ppm
contact time at 15°C =
= 150 - 15 ppm
contact time at 25°C =
= 75 - 7.5 ppm
general contact equation =
= (10 * X) - (X * ppm)
If X is mapped vs temperature:
X = (0.03786 * ((temperature)^2)) - (2.251 * (temperature)) + 40.20
Combining these equations allow calculations of halogen contact times for water temperatures from 5 to 25°C and halogen concentrations of 2 to 8 ppm.
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Specialty: Infectious Diseases